To assist persons desiring to know when structures are being prestressed and/or overstressed and/or undergoing unwanted linear deformation movement, instrumentation has been and is available. Vibrating wire strain gages designed to measure strains in steel structures, such as bridges, piles, tunnel linings, buildings, etc. are available to be installed quickly and easily. These gages utilize the so called vibrating wire principle. A steel wire of these strain gages is tensioned between two end blocks, which are secured to the surface of a steel member. The wire, when plucked, vibrates at its natural resonant frequency. This frequency, however, depends on the wire tension, which varies as the strain in the steel member varies. An electromagnet coil is used to pluck the wire and to measure the frequency of the vibration produced. Any change in frequency is transmitted from the electromagnet coil, via electrical cable leads, to be displayed as a strain change reading to be observed at a so called readout box. In a particular embodiment of this vibrating wire strain gage, a dual coil auto-resonant electrical circuit and driver is used to obtain low frequency dynamic strain measurements up to one hundred and twenty H.sub.z. The frequency signals, as necessary, may be transmitted, via electrical cable leads of electrical circuits, over many meters without loss of accuracy and precision.
There are other gages which use the so called vibrating wire principle. For example, there are vibrating wire jointmeters and crackmeters which measure displacements across joints and cracks in concrete, rock, soil, and structural members. In these meters, instruments, or monitors, a shaft is coupled to a spring, which in turn is coupled to a vibrating wire element. Movement of the concrete, rock, soil, and/or structural members causes movement of the shaft, which extends across joints and/or cracks, and is secured at one shaft end to either a portion of concrete, rock, or structural member. The other shaft end is secured to the spring, in turn secured to the vibrating wire, in turn having the housing of this vibrating wire strain gage secured to the concrete, rock or structural member on the other side of the joint or crack. Movement of the shaft, caused by movement of concrete, rock, soil, and/or structural members, on opposite sides of a joint or crack, changes the tension both in the spring and in the vibrating wire, causing a corresponding change in the frequency of vibration of the vibrating wire of the jointmeter or crackmeter. Internal ball joints are employed, as necessary, to allow for degrees of shearing motions. The frequency of vibration of these jointmeters or crackmeters is transmitted through a cable lead to a readout location. At this location the vibration frequency signal may be conditioned and displayed on a so called portable readout box, or continuously monitored by using a data logger.
Other instrumentation used to monitor possible unwanted excessive stresses and/or movements of structures centers on the use of a cantilevered beam on which strain gages are mounted. The beam is held by a ring, which in turn is positioned about a cable, such as a transmission line cable.
This available instrumentation is understood not to be applicable for use while monitoring possible unwanted excessive stresses and/or unwanted movement of flexible elongated members, which when strained tend to twist.